In recent years, high-frequency (≧30 Hz) Stirling and acoustic Stirling (or high-frequency “pulse-tube”) coolers have attracted much commercial interest because of their higher efficiency, lower maintenance, and lower noise and vibration as compared to rival technologies. One of the chief disadvantages of high-frequency Stirling coolers, however, is that the set of thermally active components (heat rejector, regenerator, and heat acceptor or “cold tip”), often referred to as the “coldhead,” is typically very intimate with the source of acoustic power that drives it. This source is usually a pressure wave generator (PWG), including one or more linear motors coupled to pistons that alternately compress and expand the working gas at the warm end of the coldhead.
On the other hand, spatial separation of coldhead and acoustic power drive permits the use of acoustic Stirling cooling in applications where space near the region to be cooled is a premium, and/or where vibration at the cold tip must be minimized. Efforts have been made to separate a coldhead and an acoustic drive. However, current technology allows only minimal separation of the drive and the coldhead, for example, the LPT9310 Stirling cooler by Thales Cryogenics BV. None of the current technologies has allowed a separation distance substantially greater than the characteristic dimension of a power wave generator, or of a substantial fraction of an acoustic wavelength (measured at operating frequency). All previous approaches use very narrow-diameter transfer lines, presumably to minimize the volume added by the transfer line, as required especially by Stirling coolers with displacer mechanisms in the coldhead, driven by the pressure wave in the working gas and demanding minimal ‘dead’ or unswept volume to preserve that driving effect. This structure tends to make the gas velocity in the tube very high, necessitating a short length to minimize the visco-acoustic losses on the tube walls. Only one patent, U.S. Pat. No. 5,794,450 (Arthur Ray Alexander), mentions the use of transfer lines to separate the PWG and the coldhead of an acoustic Stirling system for a remotely driven “pulse tube” cooler or an array of coolers. However, in Alexander, the transfer lines are much less than a wavelength in length and on the order of the pressure-wave generator dimensions. In addition, Alexander teaches a loop system, where the phase-shifting network (the acoustic equivalent of a displacer mechanism in a conventional Stirling), connected to the cold-side of the regenerator, is also connected to the PWG as a source of fluid, suggesting that a circulating, not just oscillating, flow is anticipated. Alexander is also specifically limited to the field of cooling electronic components.
One patent application publication US 20050210887, to Arman, describes a split system with the acoustic driver and coldhead separated by a transfer line for purposes of vibration isolation.
The pressure-wave generators used in acoustic Stirling coolers are often referred to as “compressors” but are not to be confused with the more familiar kind that take a steady stream of gas at a low, constant pressure and compress it to a higher constant pressure. That type of compressor is found in rival cooling technologies such as Gifford-McMahon coolers or vapor-compression refrigerators. The advantage of that type of compressor and the coolers that use them, is that the compressor and the coldhead or cold heat exchanger can be very remote from each other, with the length of separation having relatively little impact on system performance. The working fluid simply flows unidirectionally through a connecting tube or duct at low, constant velocity, incurring very little pressure drop in the process. A Stirling or acoustic-Stirling system, by contrast, is very sensitive to the size of a volume or length of a duct connecting main components because the entire system must be dynamically resonant, and every component experiences significant oscillating pressure and/or oscillating flow.
A long coupling tube which is a significant fraction of a wavelength will shift the system's resonant frequency, change the impedance seen by the pistons in the pressure-wave generator, and experience non-negligible acoustic power loss on the tube surface. At 60 Hz, the wavelength of sound in helium gas at 300K is about 17 meters; the oscillating pressure and particle velocity go through their maximum variation in a quarter wavelength, so in order to avoid wavelength effects, the length of a transfer line must be kept much shorter than a quarter wavelength. To avoid significant impacts on the stroke of the PWG motors or the PWG's natural frequency, the transfer line's total volume must also be minimized. For these reasons, Stirling and acoustic-Stirling coldheads in split systems always have had transfer lines that are extremely narrow in diameter and relatively short, ≦50 cm long for systems that run at or near 60 Hz.
To this extent, a need exists for a solution for an acoustic cooling device with a coldhead and an acoustic power source separated by a distance that is not necessarily short compared to a wavelength. This extends the usefulness of a high-frequency acoustic Stirling cooler to applications where a relatively large separation distance is required or desired.